The object of this work is the elucidation of the biochemical mechanism of compensatory skeletal muscle hypertrophy. These studies will give greater insight into the physiological regulation of normal and pathologic skeletal muscle metabolism and into the role of the nervous system in this regulation. They may also prove to be generalizable to the study of adaptation of other organs and tissues to increased physiological demand (e.g., the nervous system). Present topics of investigation include: 1) Chemical characterization of growth-factor-like activity from extracts of hypertrophied rat hindlimb muscle. 2) Chemical characterization of a similar activity from horse serum. 3) Determination of the relationship between such substances and the process of stretch-induced hypertrophy as measured by amino acid uptake and incorporation into protein, cellular content of contractile protein and cation transport.